The present invention relates to the general field of backswept blades for turbojets. More particularly, the invention relates to the geometry of blades in the fan or the compressors of a turbojet.
A turbojet is provided in particular with a fan followed, in the flow direction of the gases passing through the turbojet, by a multistage compressor. The fan and the compressor are elements of the turbojet through which the gas flow passes. Each of them comprises a row of moving blades that are circumferentially spaced apart from one another so as to define passages for the gas flow. The blades of these elements are subjected to speeds of rotation that can generate subsonic to supersonic speeds in the flow of gas passing through these elements of the turbojet. Although high flow speeds make it possible in particular to improve the mass flow rate of the gas, thereby increasing the thrust of the turbojet, they can also sometimes present the drawback of generating high levels of noise. In particular, the “supersonic bang” corresponding to the gas flow switching from supersonic speeds to subsonic speeds contributes to a large fraction of such noise. Other interaction phenomena involving turbulence of the gas flow in the vicinity of the fan (broadband noise) also constitute sources of noise in the fan.
Engine manufacturers thus seek to devise fan and compressor blades that enable turbojet thrust to be increased while minimizing the noise generated by the flow of gas passing through the fan or the compressor. In addition, when designing such blades, various other parameters need to be taken into account such as the aerodynamics and the strength of such blades. Blades need to be designed so as to optimize the mass flow rate and the compression of the gas going past them, while guaranteeing good mechanical strength for the blades. In particular, at high speeds of rotation, the mechanical stresses to which the blades are subjected are more severe because of the high level of vibration and the centrifugal force that are applied to the blades.
Numerous geometries have been proposed for fan and compressor blades. They are characterized mainly by their stacking relationships for blade sections, their general curvature, and the possible presence of aerodynamically swept-back portions for improving aerodynamic performance and for reducing the noise generated by the fan and the compressors of the turbojet. However, none of those blades enable aerodynamic operation to be obtained that is efficient under all conditions of turbojet use, in particular both at high speed or high revolutions per minute (rpm) as occurs for example during takeoff and when the airplane reaches its maximum altitude, and at partial rpm, as happens for example during an airplane approach stage, and while also satisfying noise standards that are becoming more and more restrictive.